1. Field of the Invention
The present invention relates to wavefront-aberration-measuring devices and exposure apparatuses.
2. Description of the Related Art
To photolithographically manufacture semiconductor devices and the like, projection exposure apparatuses are typically used in each of which a pattern formed on a reticle is transferred to a wafer by exposure through a projection optical system. With a demand for semiconductor devices having finer circuit patterns, commercialization of projection exposure apparatuses employing extreme ultraviolet (EUV) light having a wavelength of about 5 to 20 nm, which is shorter than the wavelength of ultraviolet light, has been under consideration.
To accurately transfer patterns on reticles to exposure targets at specific magnifications, projection optical systems are required to have high imaging performance with less aberration. With the demand for semiconductor devices having finer circuit patterns, transfer performance is becoming more sensitive to aberrations caused by projection optical systems. Therefore, wavefront aberrations caused by projection optical systems need to be measured accurately.
Exemplary techniques for accurately measuring wavefront aberrations of projection optical systems for EUV apparatuses include a point diffraction interferometer (PDI) and a shearing interferometer. Shearing interferometers do not require highly advanced alignment technology compared with PDIs.
A well-known technique of a shearing interferometer intended for wavefront aberration measurement is disclosed in Japanese Patent Laid-Open No. 2005-159213, in which a spherical wave that is output from a single pinhole provided in an object plane of an optical system to be tested (hereinafter referred to as to-be-tested optical system) is made to enter the to-be-tested optical system. The wavefront of light output from the to-be-tested optical system is deformed with aberration caused by the to-be-tested optical system. In addition, a diffraction grating provided below the to-be-tested optical system splits the light output from the to-be-tested optical system into diffracted beams of different orders. In general, in a shearing interferometer of this type, such as the one disclosed in Japanese Patent Laid-Open No. 2005-159213, since diffracted beams having intensities sufficient for measurement need to be produced through the single pinhole provided in the object plane, the pinhole needs to be illuminated with high illuminance. Therefore, light from a high-brightness light source needs to be collected into the pinhole.
Examples of such a light source currently available in general include an undulator light source provided in an electron storage ring. Undulator light sources, however, are of very large sizes and are therefore not practical for use in assembly lines and installation sites of exposure apparatuses. Considering such a situation, it is desirable that an exposure light source having a small size be also used as a light source for wavefront measurement. Such a configuration is realized in Japanese Patents Laid-Open No. 2006-332586 and No. 2007-234685, for example. Specifically, a number of pinholes or apertures are provided in the object plane in a specific arrangement, whereby the efficiency of light utilization is improved. Consequently, wavefront aberration caused by the projection optical system can be measured with a shearing interferometer and with a low-brightness light source, such as a plasma light source, intended for exposure.
In a typical shearing interferometer, light transmitted through a to-be-tested optical system contains a wavefront aberration caused by the to-be-tested optical system. Moreover, when the light is diffracted by a diffraction grating, another wavefront aberration is added. Hereinafter, the wavefront aberration caused by the diffraction grating is referred to as diffraction-grating aberration. Diffraction-grating aberration occurs because light incident on the diffraction grating is not composed of parallel beams but convergent or divergent beams, and because of errors in the orientation of the diffraction grating and manufacturing errors in the periodicity of the diffraction grating.
Diffraction-grating aberration leads to errors in measurement of the wavefront aberration of a to-be-tested optical system. To realize highly accurate wavefront aberration measurement, such diffraction-grating aberration needs to be eliminated. An exemplary calibration technique applicable to the shearing interferometer disclosed in Japanese Patent Laid-Open No. 2006-332586 is disclosed in Japanese Patent Laid-Open No. 2008-198799, in which a number of extremely small pinholes are provided in the image plane so that light transmitted through the pinholes is diffracted and produces spherical waves free from aberration.
It is expected that there will be a demand for projection optical systems of exposure apparatuses having large numerical apertures (NA) with increasingly finer transfer patterns. Accordingly, the diameter of the pinholes that produce spherical waves as disclosed in Japanese Patent Laid-Open No. 2008-198799 will become extremely small so as to realize a large NA. In general, a pinhole that produces an aberration-free spherical wave from light transmitted therethrough is considered to have a diameter expressed as D≦λ/(2NA). If NA is 0.3 and the wavelength λ is 13.5 nm, D is 22.5 nm or smaller. It is not easy to manufacture pinholes of such a small size. Besides, the ratio of the thickness of the pinhole to the diameter of the pinhole, i.e., the aspect ratio, becomes large, resulting in a low transmittance of the pinhole. This is disadvantageous in that a sufficient amount of light for wavefront measurement cannot be produced. Consequently, the wavefront aberration of the to-be-tested optical system cannot be measured with high accuracy.